This invention relates generally to multilayer coils and, more particularly, is directed to a multilayer coil for use with a DC motor.
It is known in the art to construct a flat brushless DC motor which generally includes at least one pair of permanent magnet north and south pole pieces secured to the rotor, and corresponding energizable coils mounted on the stator in opposing relation to the permanent magnet pole pieces, with each coil having conductor segments for carrying current in directions which are normal to the magnetic flux generated by the permanent magnet pole pieces. With such an arrangement, the rotor is caused to rotate relative to the stator by a torque produced by the interaction of magnetic flux generated by the magnet pole pieces and the current flowing through the coils, with the direction of rotation being perpendicular both to the direction of magnetic flux and the direction of current flow.
In flat brushless DC motors, each coil may be formed with a plurality of layers, that is, as a multilayer coil having a substantially constant or uniform thickness so that the air gap between each coil and the permanent magnet pole pieces is also substantially constant or uniform. It is to be appreciated that for multilayer coils, each coil is formed of a continuous conductor wire so as to have a first or beginning lead wire and a second or terminating lead wire, the first one of which terminates at the inner periphery of the respective multilayer coil and the second one of which terminates at the outer periphery of the respective coil, the first and second lead wires being supplied with a sinusoidal current for energizing the coil. However, in order to supply the sinusoidal current to both lead wires, the first lead wire is generally led from the inner periphery of the coil to the outside of the coil. It is to be appreciated that the first lead wire must therefore cross over the coil winding, and this crossing over occurs in the air gap between the respective coil and the permanent magnet pole pieces. Since the gap is required to be of a constant or uniform width and of a relatively small dimension, the crossing over of the first lead wire in the gap results in non-uniformity of the gap width and requires the gap width to be increased, which results in a decrease in the efficiency of the motor.
Further, the first lead wire must be adhered to the surface of the coil in the gap by an adhesive agent to prevent centrifugal force of the motor from moving the lead wire, possibly into contact with the rotor magnet. The use of such adhesive, however, results in further non-uniformity of the gap width so that the distance between the rotor and stator must be still further increased.
It has therefore been proposed to provide each multilayer coil on a printed circuit board with the first inner lead wire being soldered to the printed circuit board at the interior of the coil. In this manner, the first lead wire does not cross over the coil, and therefore, does not require an increase in the gap width. It is to be appreciated, however, that the inner area of each coil is relatively small, and accordingly, soldering of the coil to the printed circuit board may result in damage to the coil as a result of the intensive heat thereat.